1. Field of the Invention
This invention relates in general to optical systems that spatially homogenize light produced by non-homogenous optical sources. In particular, it relates to methods and systems that utilize spatially homogenizing light pipes in combination with optics with positive optical power to produce a light output that is uniform both in intensity and irradiance from a single source or an array of light-emitting diodes. The substantial spatial and possibly spectral non-uniformity in the input aperture of the optical system introduced by this type of source generally translates into non-uniformity in the output of traditional light collection systems.
2. Description of the Related Art
Light-emitting diodes (LEDs) have already gained popularity in various applications as versatile optical sources possessing long life, high energy efficiency, and small spectral bandwidth (typically 20 to 30 nm, measured as full-width-half-maximum). However, it is traditional broadband incandescent or arc sources such as quartz halogen or xenon lamps that continue to be predominantly used for object illumination. On one hand, this is due to the high brightness of these sources, which satisfies the stringent requirements of such applications as manufacturing, medical, military, or machine vision. On the other hand, it is due to the fact that the broad spectra of these sources match better the approximately 300-nm range of the spectral response of the optical receivers used in a typical imaging system (such as the sensor array of a digital system, or even the human eye). The above notwithstanding, the use of LEDs for object illumination would be definitely preferred if it were possible to broaden the spectral band and increase the brightness of LED light delivered to the object without sacrificing the color, spatial, or polarization uniformity typically associated with traditional sources.
The first need, to enhance the spectral range of LED-borne light used for illumination, has been addressed in the art in two major ways—by the use of wavelength conversion phosphors with a single LED source and by the use of several LEDs emitting in different spectral regions. According to the first approach, a blue- or UV-light-emitting diode is coated with a wavelength conversion phosphor that absorbs a fraction of the LED light and re-emits it at longer wavelengths, thus effectively broadening the overall emission spectrum. (The gamut and degree of spectral uniformity of re-emitted light is controlled, for example, by judiciously modifying the phosphor content.) The second approach, shown schematically in FIGS. 1(A)-(C), combines several LED sources generating light in different spectral bands (for example, red, green, and blue, identified as R, G, and B, respectively) and provides optical means for collecting and multiplexing all the light to create a mixed-color light output. Depending on the application, this “color mixing” scheme may utilize three, four, or more spectrally different types of LEDs. Certain spectral shortcomings, such as variations and changes in the spectral distribution of the LED outputs occurring as a result of differential aging and temperature effects, are typically addressed electronically by providing an appropriate feedback loop or a temperature compensation scheme.
The second need, to increase the brightness of the light produced by a single LED or an LED-array source and delivered to the object, may be met by an appropriate light-collecting and illumination optical system designed to reduce the angular spread of the light to meet the application needs without unduly reducing the inherent brightness of the source. Still, the distribution of the light output from LED sources using conventional optical means is known to lack homogeneity of both irradiance and intensity. Sources other than LEDs may pose similar or worse irradiance and intensity non-uniformity challenges, as well as possibly polarization conditions that will make the source perform differently than traditional sources if not redistributed by the collection system. Although preserving brightness is a necessary feature of a collection system, high performance with respect to brightness conservation is not enough if the resulting output contains significant elements of the various types of possible non-uniformities found at the input. Although the specific impact of non-uniformity in the output varies, the effect is generally undesirable and performance limiting for most visual- and sensor-based applications.
As understood in the art, the terms “irradiance” and “intensity” are used to describe the distribution of light, and are defined as complementing terms expressed in Cartesian (rectilinear) and spherical (angular) coordinates, respectively. Accordingly, for the purposes of this disclosure the terms “irradiance” or “surface density” are used to signify the flux of radiant energy flowing across a unit area of real or imaginary surface. The terms “intensity” or “angular density,” on the other hand, refer to the flux of radiant energy per unit of solid angle propagating in a given direction.
While non-uniformities from a narrow band source would be objectionable in many applications, the problem of non-uniformity of irradiance and intensity of light output is particularly pronounced when arrays of spectrally different LEDs (such as the RGB arrays of FIG. 1) are used for broadband illumination of objects. The problem is manifested in the fact that any mismatch in the irradiance or intensity profile in the light output produced by each individual LED produces a non-uniform color distribution in the viewing plane (or in the detector plane). For example, a simple RGB LED array coupled into a single optical fiber cable to form a fiber illuminator may produce an output that is perceived as a whitish central spot surrounded by one or more rings that are distinctly tinted in favor of one of the input colors. The additional degree of non-uniformity of such a mixed-color LED array (versus a single color array with only spatial non-uniformities) within a single light-collecting system only aggravates the problem of non-uniformity in the output. Although the degree and impact of such color non-uniformity depend on several factors (such as spectral bands, number and arrangement of LEDs used, the particular optical scheme, and the application), this effect is nearly universally undesirable. For instance, the image of a multi-colored object illuminated by such an optical source will not accurately reproduce the coloration of the object and, therefore, will convey erroneous optical information. Even in monochromatic imaging applications (such as in machine vision) the spatial color non-uniformity of the illumination source will result in perceiving a uniformly colored object as having gray-level variations due to the variable spectral response of the optical receiver.
One common approach to reducing spatial non-uniformity of either single- or mixed-color light distribution utilizes a so-called integrating light pipe (ILP) 12, such as a polygonal cross-section integrating light pipe (PCIL), as schematically shown in FIG. 2. The ILP 12 operates by mixing light inputs 14 received from a mixed-color array, multiplexed input, or from a discrete color light source 16 (such as any of the mixed-color LED array sources of FIG. 1, for example) and partially blending the radiation of different colors to provide a uniform irradiance profile in the output plane 18. (LED-based projection displays currently available on the market employ a similar system that incorporates a rectangular cross-section integrating light pipe and different color LED arrays, packaged independently within separate collecting optical components.) FIGS. 3(A) and (B) illustrate the irradiance of a monochromatic beam at the entrance and exit planes, respectively, of a typical conventional hexagonal integrating light pipe 12 (as shown schematically in FIG. 2), which clearly show the homogenizing effect of the light pipe on the irradiance of a beam of light. Similarly, FIGS. 4(A) and (B) illustrate the intensity of the same monochromatic beam at the entrance and exit planes, respectively, of the light pipe 12, showing the corresponding lack of homogenizing effect of the pipe on intensity of the light beam. Similar results can be shown for multi-spectral light. Thus, while ILPs may be used to provide a uniform irradiance distribution of light, any non-uniformity of intensity remains substantially non-uniform and a problem that is yet unsolved. This deficiency of conventional ILP-based illumination systems makes mixed-color LED arrays, packaged within a single collecting optic, unsuitable for all but very low-end illumination applications. As the light propagates away from the output plane, significant color and general irradiance non-uniformities will likely be present along the path due to the unresolved differences in the angular distribution.
Therefore, there exists an unresolved problem in the angular uniformity of the distribution of the light produced by both narrow band and mixed-color sources (such as single or multi-color LED arrays). Resolving this problem is critically important for correct monochromatic or polychromatic imaging, which implies illuminating the object with light that has both uniform irradiance and uniform intensity. This invention solves this problem by adding a two-stage ILP system to a multiplexed-color LED-array light source, where the two independent ILP-stages are optically connected by an element possessing optical power, and each stage is positioned one focal length away from the principal plane of the optical element. The first stage of the ILP system provides uniform irradiance, while the combination of the optical element and the second ILP stage corrects non-uniformities in the intensity of the light. As a result, the ILP system of the invention produces overall spatial uniformity in the light propagating through the system and provides an unpolarized narrow band or polychromatic output that is homogeneous both in intensity and in irradiance independent of the various non-uniformities of the source. The preferred optical element used to that end is a reflector in order to avoid chromatic aberrations.